The present invention relates to a screening device according to the preamble of claim 1, as is known from DE 964 286 C.
Such pulper devices are disclosed in DE-AS 23 48 540 and in DE-AS 26 58 845 as well as in DE 20 61 519 C2 and in U.S. Pat. No. 2,661,666. Another pulper device is disclosed in FR-A-1 374 767.
Screening devices are used in most various industrial branches for the most different cases of application. Screening devices are widely used in processing technologies for sorting free-flowing substances, which contain solid materials of different sizes. An important case of application relates to the so-called pulper, which is used for processing material mixtures containing easy to pulp components. Important cases of application relate to the processing of wastes and waste-like materials as well as to paper industry for the processing of waste paper.
Such a pulper comprises a vessel provided with a rotor. During operation the material mixture to be processed is charged into the vessel, and a liquid, in general water, is moreover filled into the vessel. The rotor is rotated thereby generating strong flow forces. Seen in an axial section through the pulper, said flow has a toroidal form, whereby two main flow components overlap each other, namely a radial and a concentric flow direction. In the region of the rotor said flow at first flows outwardly against the wall of the vessel in a radial manner and then rises upwardly, so as to flow again radially towards the inside in the region of the surface of the liquid, whereupon it is conducted to the center of the rotor. Said flow can be supported by mounting additional guiding plates. At the same time, the liquid is subjected to a motional impulse in the rotational direction of the rotor. Said toroidal flow causes the solid matter contained in the liquid to repeatedly reach the region of the rotor, where it is processed.
Due to the strong flow the easily pulpable solid materials contained in the pulper are pulped. Apart from the pulping, partly also a shortening of fibers takes place or, respectively, part of the pulpable materials is dissolved. As the main emphasis of the process, however, lies in the pulping, merely the xe2x80x9cpulpingxe2x80x9d will be discussed in this respect, while the disintegration and the dissolution taking place parallel to it will not be dealt with separately.
The pulped components of the charged material mixture form a suspension together with the liquid, which can be drawn off from the pulper by means of a punched screen. The non-pulped components remaining in the pulper (hereinafter also called xe2x80x9ccoarse materialsxe2x80x9d) can be removed from the pulper in a different manner, e.g. by means of a coarse dirt collector projecting into the pulper.
A preferred case of application, for instance, relates to the processing of waste paper or the processing of wastes for a biological utilization of individual components. During the processing of wastes the components, which under a biological aspect are favorable for utilization, are separated from those components, which cannot biologically be utilized, as the biologically utilizable components can be pulped easily whereas it is difficult or impossible to pulp the other components.
The formed suspension separated by the punched screen, in which the biologically utilizable components are concentrated, can then be subjected to the biological processing, whereas the biologically non-utilizable components, i.e. the coarse materials remaining in the pulper after the suspension has been drawn off, are separated and can be supplied to another appropriate form of utilization, or they may be removed.
Other screening devices so far used in connection with pulpers for separating coarse materials from the suspension are either formed as flat screens (such as in EP 0 520 172 B1 or DE 195 06 084 A1) or as cylindrical screens (such as in EP 0 598 187 B1).
The use of a flat screen results in that the toroidal flow is diverted on the screen entailing the disadvantage that the solid materials in the pulper, which cannot or only with difficulties be pulped, such as pebbles, broken glass or the like, thereby cause a high wear of the screen. Moreover, there is the risk that the screen holes are clogged. Heavy components in the material mixture tend to sink onto the screen, where they remain unless they are pulped, so that they cannot pass the screen together with the suspension. A common method for keeping the screen free from such coarse materials and for guaranteeing the passage of the suspension is to repeatedly clean the surface of the screen by means of clearance rails. Said clearance rails, however, are also subject to a high wear.
The use of a cylindrical vertical screen entails other disadvantages. The suspension is here drawn off through the vertically situated screen elements. The non-pulped coarse materials in this case tend to accumulate around the screen on the bottom of the pulper thereby clogging the passageway through the screen. The flow conditions in the pulper are not suited to effect a satisfactory progression of the coarse materials and thereby a cleaning of the surface of the screen, since the flow does not touch the vertical elements of the screen. Here, too, the screen can only be cleaned by additional methods, in general by using clearance rails. The coarse materials accumulating around the screen on the bottom of the pulper likewise contribute to a high wear. The use of clearance rails, moreover, results in another essential disadvantage, as the clearance rails strongly disintegrate the plastic components present in the waste. Said small plastic particles can thereby pass the punched screen together with the suspension. In this way they contaminate the suspension destined for a biological utilization, which also results in an undesired contamination level of the finished products, above all of the compost and the process water.
It is the object of the present invention to improve the above-defined pulper device for separating coarse materials from a suspension so as to reduce wear, to facilitate the cleaning and maintenance of the screen, to accelerate the passage of the suspension and to improve the product quality.
According to the invention said object is provided by the pulper described in patent claim 1. The subclaims describe additional preferred embodiments of the invention.
The gist underlying the present invention resides in that a screening device is used, which has the shape of a truncated cone. The truncated cone points upwardly with the smaller circular surface thereof. The holes of the screening device are provided on the inclined lateral surface of the truncated cone. The inclination is configured such that it lies in an approximately tangential or, respectively, parallel manner to the toroidal flow. Moreover, the screening device is segmented and can be mounted externally.
The angle or inclination amounts to preferably 30xc2x0-50xc2x0, whereby said value can vary in response to the shape of the vessel. The inclined arrangement results in that the flow constantly causes the coarse materials sinking down in the pulper to be removed from the lateral surface of the screen, i.e. they are entrained by the flow. Said process is even supported by the use of scrapers, which are, however, subject to a smaller load in comparison to those according to the prior art, as the cleaning is not exclusively effected by clearance railsxe2x80x94in contrast to the described flat screens or cylindrical screensxe2x80x94but to a considerable extent by the flow.
As the coarse materials are constantly moved away from the screen with the flow so that, in contrast to the screening device used according to the prior art, they can accumulate on the screen or around the screen to a much smaller extent, also the abrasion caused by sand, pebbles, glass etc. is essentially reduced. Due to the better cleaning of the surface of the screen, moreover, the passage through the screen is rendered more favorable, whereby the draw-off time of the suspension is reduced.
The holes in the screen may be shaped differently, they may have, for instance, square, circular or oblong shapes. Their characteristic dimensions preferably are 6-30 mm. The screen is preferably sub-divided into segments so as to facilitate the disassembly thereof.
The scrapers used for supporting the cleaning of the screen holes from coarse dirt are preferably mounted on the rotor or the shaft thereof and may likewise have different shapes. They may, for example, be straight or inclined. The scrapers are preferably continuously or discontinuously curved opposite to the flow, and are provided with a wear shield. The wear shield can preferably be exchanged and/or adjusted.
For drawing off the suspension from the pulper through the screen a pump is preferably applied, the drive of which can be controlled. However, also an unobstructed discharge into a lower-lying storing vessel is possible.
The invention is especially suited for the use in processing and separating waste mixtures with a high organic portion, e.g. of communal biological wastes. Apart from biogenous organic components, which are suited for a biological treatment and subsequent recycling into the natural circulation, said wastes are characterized by a relevant portion of non-biogenous organic interference materials, such as plastics, pebbles, glass and metals.
The present invention will hereinafter be explained in more detail by means of a preferred embodiment with reference to the accompanying drawing.